Electrosurgical instruments and systems comprising an active and return electrode and powered by a radio-frequency voltage supply as is illustrated for example in FIG. 1, are widely used in procedures for treating target tissues in the body. Treatment of the target tissue involves placing the electrodes (10) in close proximity to a target tissue (12) and applying power to the electrodes to cause Coblation®, heating, ablation, coagulation, cutting, removal, puncturing, probing, and otherwise stimulating the tissue. In some systems an electrically conductive fluid is supplied between the electrodes to generate plasma to treat the tissue; in other systems, the body's fluids are used as the conductive fluid. An example of such system for treating tissues with plasma is described in commonly assigned U.S. patent application Ser. No. 10/661,118, now U.S. Pat. No. 7,276,063, hereby incorporated herein by reference for all purposes.
In an electrosurgical system as illustrated in FIG. 1, the electrodes are located on the distal end portion of the shaft (14). In one configuration of the distal end portion of the shaft as is illustrated in detail FIG. 2, the return electrode (16) is positioned on the outside perimeter of the shaft and, in various embodiments, surrounds the active electrode (18) which is within the shaft. To ensure that plasma (20) generated on the active electrode is closest to the tissue, the distal tip of the active electrode projects beyond the return electrode. Also, in the embodiment illustrated in FIG. 2, the active electrode is separated from the return electrode by an insulator (24), and electrically conductive fluid (26) is supplied between the electrodes by a fluid lumen (27) circumferentially positioned on the shaft around the return electrode. This conductive fluid as is illustrated in FIG. 2 forms a conductive fluid pathway (38) between the electrodes
Also in an electrosurgical system as is illustrated in FIG. 2 and as will be appreciated by one ordinarily skilled in the art, when power is applied across the electrodes, an electric field (22) sometimes in the order of 30,000 V/cm is generated which, for some procedures, is not desired as these fields can interact with the tissue and cause electrical stimulation of neuromuscular structures (28) within the tissue.
Accordingly, there is a need for apparatus and systems for use in electrosurgical procedures wherein unwanted electrical stimulation of the tissue is avoided, and which can be used in confined spaces within the body.